1. Field of the Invention
The present invention relates to a digital data transmitting apparatus for transform coding a digital image signal by, for example, two-dimensional cosine transform (discrete cosine transform) to compress the amount of data and, more particularly to a digital data transmitting apparatus suitable for compressing the amount of transmission data.
2. Description of the Prior Art
To suppress the redundancy of an image signal, there has been known a transform coding technique for dividing a screen into blocks consisting of a particular number of pixels and performing linear transform of each block with a transform axis corresponding to the property of the original image signal. Examples of transform coding techniques are Hadamard transform, cosine transform, and so forth. In a conventional cosine transform coding apparatus, image data is divided into blocks consisting of (8.times.8) pixels and each block is cosine transformed (DCT transformed). Thus, (8.times.8=64) coefficient data is formed.
The coefficient data is output by the zigzag scanning from a DC component to high frequency components and then requantized. The requantized output signal is sorted in the order of absolute values of amplitudes. Thereafter, the amplitudes and addresses are differentiated. The differential signal is encoded into a code signal consisting of a particular number of bits by run-length coding technique or Huffman coding technique. This code signal is supplied to a buffer memory. The buffer memory is provided so as to transform the transmission rate of a code signal generated in a variable length coding process into a rate which does not exceed the rate of the transmission line for use. Although the input data rate of the buffer memory is variable, the output data rate thereof is nearly constant. The buffer memory detects the variation of the amount of transmission data. In response to the detection, the quantizing step in the requantizing process is controlled. In addition, the coefficient data to be transmitted is controlled so that it has a particular amount of data by a threshold operation in the variable length coding process. The threshold operation is a process for subtracting a predetermined threshold value from coefficient data whose absolute value is larger than the threshold value. However, the coefficient data of the DC component is excluded from the threshold operation.
In the conventional DCT transform, when the amount of information is compressed, while obtaining a particular bandwidth of images, fixed amount of information such as the DC component should be transmitted for all blocks. Since the fixed amount of information is not small, the ratio of data compression cannot be satisfactorily improved.